THE CEREBRUM, AND ITS FUNCTIONS. 825 



waking state decrease, a smaller amount of sleep suffices ; or, if the slumber be 

 protracted, it is usually less deep and refreshing. It may be noticed, however, 

 that very old persons usually pass a large proportion of their time in sleep, or 

 rather in a sort of heavy doze, especially after meals ; as if, in consequence of 

 the want of energy of their nutritive operations, a very long period of repose is 

 necessary to repair the waste which takes place during their short period of 

 activity. In regard to the influence of temperament, it may be remarked that 

 a plethoric habit of body, sustained by full diet, usually predisposes to sleep, 

 provided that the digestive powers be in a vigorous condition ; persons of this 

 constitution frequently pass nine or ten hours in slumber, and maintain that 

 they cannot be adequately refreshed by less. On the other hand, thin wiry 

 people, in whom the "nervous" temperament predominates, usually take com- 

 paratively little sleep, notwithstanding the greater activity of their nervous 

 system when they are awake ; but their slumber, while it lasts, is generally 

 very deep. Persons of "lymphatic" temperament, heavy passionless people, 

 who may be said to live very slowly, are usually great sleepers ; ' but this rather 

 because, through the dulness of their perceptions, they are less easily kept 

 awake by sensorial or mental excitement, than because they really require a 

 prolonged cessation of activity. As they are half asleep during the waking 

 state, so would it appear that the constructive operations must be far from 

 active while they are asleep, so little do they seem restored by the repose. The 

 amount of sleep, caeteris paribus, required by individuals, is very greatly 

 influenced by habit; and, contrary to what we might anticipate, we find that 

 the briefest sleepers have usually been men of the greatest mental activity. 

 Thus Frederick the Great and John Hunter are said to have only required five 

 hours' sleep out of the twenty-four. General Elliot, celebrated for his defence 

 of Gibraltar, is recorded not to have slept more than four hours out of the 

 twenty -four. It may be doubted whether it would be possible for any one to 

 sustain a life of vigorous exertion upon a smaller allowance than this; and the 

 general fact is, that from six to eight hours of repose, out of every twenty-four, 

 are required to keep the system of an adult in a state of healthful activity. 

 The influence of habit may be brought to bear on the protraction, as well as on 

 the abbreviation, of the usual period. Thus Quin, the celebrated actor, could 

 slumber for twenty-four hours successively; and Dr. Reid, the metaphysi6ian, 

 could take as much food, and afterwards as much sleep, as were sufficient for 

 two days. It is needless to dwell upon the obvious fact, that, other things 

 being equal, the amount of sleep required by man is proportional to the amount 

 of mental exertion put forth during the waking hours; since this is an obvious 

 result of what has been laid down as the cause of the demand for sleep. It 

 may be remarked, however, that we must not measure the amount of sleep by 

 its duration alone; since its intensity is a matter of equal importance. The 

 light slumber which is disturbed by the slightest sounds cannot be as renovat- 

 ing as the profound sopor of those whom no ordinary noise will awake. 



845. There are certain states of the encephalic centres, in which there is an 

 entire absence of sleep; and this may continue for many days, or even weeks or 

 months. Insomnia is, for instance, one of the characteristics of acute mania, 

 and may also exist in various forms of monomania ; it is usually, also, one of 

 the symptoms of incipient meriingeal inflammation; and it may constitute a 

 specific disease in itself. In all these cases, however, the preponderance of the 

 destructive processes over the constructive manifests itself, sooner or later, in 

 the exhaustion of the mental and bodily powers. Thus mania, when prolonged 

 or frequently occurring, subsides into dementia; and, if it continue for any 

 length of time, is sure to be followed by a great sense of wretchedness and 

 prostration, frequently accompanied by continual restlessness. Such effects, too, 

 in a less aggravated degree, result from habitual deficiency of sleep; whether 



